Abstract. Experimental and theoretical uncertainties in the measurement of cloud
condensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counter
from Droplet Measurement Technologies (DMT-CCNC) have been assessed by model
calculations and calibration experiments with ammonium sulfate and sodium
chloride aerosol particles in the diameter range of 20–220 nm. Experiments
have been performed in the laboratory and during field measurement campaigns,
covering a wide range of instrument operating conditions (650–1020 hPa
pressure, 293–303 K inlet temperature, 4–34 K m−1 temperature
gradient, 0.5–1.0 L min−1 flow rate). For each set of conditions, the
effective water vapor supersaturation (Seff, 0.05–1.4%) was
determined from the measured CCN activation spectra (dry particle activation
diameters) and Köhler model calculations. High measurement precision was
achieved under stable laboratory conditions, where the relative standard
deviations of Seff were as low as ±1%. During field
measurements, however, the relative deviations increased to about ±5%,
which can be mostly attributed to variations of the CCNC column top
temperature with ambient temperature. The observed dependence of Seff
on temperature, pressure, and flow rate was compared to the CCNC flow
model of Lance et al. (2006). At high Seff the relative deviations
between flow model and experimental results were mostly less than 10%, but
at Seff≤0.1% they exceeded 40%. Thus, careful experimental
calibration is required for high-accuracy CCN measurements – especially at
low Seff. A comprehensive comparison and uncertainty analysis of the
various Köhler models and thermodynamic parameterizations commonly used
in CCN studies showed that the relative deviations between different
approaches are as high as 25% for (NH4)2SO4 and 12% for NaCl.
The deviations were mostly caused by the different parameterizations for the
activity of water in aqueous solutions of the two salts. To ensure
comparability of results, we suggest that CCN studies should always report
exactly which Köhler model equations and parameters were used. Provided
that the Aerosol Inorganics Model (AIM) can be regarded as an accurate source
of water activity data for highly dilute solutions of (NH4)2SO4 and
NaCl, only Köhler models that are based on the AIM or yield similar
results should be used in CCN studies involving these salts and aiming at
high accuracy. Experiments with (NH4)2SO4 and NaCl aerosols showed
that the conditions of particle generation and the shape and microstructure
of NaCl particles are critical for their application in CCN activation
experiments (relative deviations up to 18%).